1. Field of the Invention
The present invention relates to apparatus and method for fabricating thin films. More specifically, the present invention relates to a method and apparatus for measuring the thickness of thin films.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
A silicon-on-insulator (SOI) semiconductor wafer typically includes a sandwich structure fabricated by growing a silicon dioxide film on one surface of each of two silicon wafers and bonding the two silicon dioxide film surfaces together at high temperature. Other materials such as, for example, silicon nitride, may be used for the insulator material and other materials may be used for the wafer material. One of the two outer silicon surfaces of the sandwich structure is typically mechanically ground and polished to an average thickness of several microns. This mechanical process unfortunately results in large spatial variations in the thickness of this outer silicon layer over the surface of the wafer. To reduce these spatial variations, a thickness error map that indicates thickness non-uniformities of this outer silicon layer over the entire wafer surface, is typically required for subsequent manufacturing operations, e.g. micro-polishing. The steps of measuring the spatial variations in the thickness of the outer silicon layer followed by thinning and smoothing the surface by micro-polishing may need to be performed several times before the entire outer silicon layer achieves the desired thickness. In order to reduce costs and increase production, a measurement of at least 400 points on a wafer surface in 60 seconds is desirable.
Current commercial instruments, however, typically provide film thickness measurements at only a single point on a surface. These instruments use a focused lens or a fiber bundle to locally illuminate the film surface with a beam of monochromatic light. A grating or prism spectrograph is typically used to measure the surface spectral reflectance at each point. This surface spectral reflectance data must be numerically corrected due to variations in the angle of incidence caused by the illuminating beam f-number.
These commercial instruments may be extended to cover the entire wafer surface by moving either the measuring instrument or the wafer in a controlled manner. However, the time required for these instruments to determine the thin film layer thickness at a single point is on the order of one minute. Further, characterizing an entire film surface of at least 400 measurement points far exceeds the time typically afforded for efficient wafer production.
The need in the art for a faster system and technique for measuring the thickness of thin films is addressed by the invention of two copending U.S. Patent Applications entitled, APPARATUS AND METHOD FOR MEASURING THE THICKNESS OF THIN FILMS, Ser. No. 07/804,872 filed Dec. 6, 1991 and Ser. No. 07/987,926 filed Dec. 10, 1992, by Anthony M. Ledger and assigned to the present Assignee. This application discloses an electro-optical imaging system for efficiently determining a thin film layer thickness of, for example, a wafer over a full aperture. Non-uniformities in this layer thickness are obtained by measuring the reflectance characteristics for a full aperture of a wafer surface and comparing this measured reflectance data to reference reflectance data by using numerical iteration or by using a calibration wafer having known layer thicknesses.
To efficiently measure the reflectance characteristics of a wafer layer, a filtered white light source is used to produce a sequence of collimated monochromatic light beams at Several different wavelengths. These collimated monochromatic beams are individually projected onto the entire surface of the wafer, and coherent interactions occur between this light as it is reflected from the physical boundaries in the wafer structure. As a result of these interactions an interference fringe pattern is formed on the surface of the wafer for each projected beam and, consequently, for each wavelength. A reflected image of each fringe pattern is projected onto a detector array of, for example, a charge coupled device (CCD) camera, where the full aperture of this image is then captured. The fringe pattern image is captured by digitizing pixels in the CCD camera detector array corresponding to the image present. A reflectance map of the entire wafer surface is generated from this captured fringe pattern image. Several reflectance maps are generated from each measured wafer to eliminate thickness ambiguities which may result from outer layers having phase thicknesses greater that 2.pi..
The reference reflectance data for a wafer may be obtained by a theoretical calculation or through the use of a calibration wafer. The theoretical method consists of numerically computing reference reflectance characteristics based on assumed values for the intrinsic optical properties of the wafer materials. Alternatively, a calibration wafer, having a known thickness profile, may be constructed from the same batch of materials used to construct the wafer to be measured. By subjecting this calibration wafer to the measuring method of the present invention, reference reflectance data is obtained for the known wafer.
The comparison between the measured reflectance data and the reference reflectance data can then be performed by a computer. The computer can then provide a mapping of layer thickness or a mapping of layer thickness non-uniformities over a full aperture of the wafer.
While this invention represents a substantial advance in the state of the art, it is limited in the measurement of multi-layer film stacks. For multi-layer film stacks, film stack reflectivity is measured as a function of incident wavelength for a large number of physical locations. The "signature" of reflectivity vs. wavelength is then compared to a "library" of such signatures, each generated from theoretical predictions for a slightly different thickness of one layer in the stack. The library is essentially a column of data based on a theoretical prediction of the thickness of one layer. The library is normally generated with a fixed thickness for all other films in the stack. The best match to this library is determined by minimizing to the least squares error.
Using this technique, measurement of the thickness of two layers of thin films requires that the thickness of one layer be known along with the index of refraction `n` and the imaginary index of refraction `k` for both layers. Unfortunately, in many applications, there is a need in the art for a system and technique for measuring the thickness of two adjacent thin film layers in a multi-layer film stack where the thickness of both layers is unknown.